1. Field
This invention relates in general to nuclear reactor systems, and, in particular, to in-core instrumentation for such systems, that pass through the upper internals of the reactor pressure vessel.
2. Related Art
In a nuclear reactor for power generation, such as a pressurized water reactor, heat is generated by fission of a nuclear fuel such as enriched uranium, and transferred to a coolant flowing through a reactor core. The core contains elongated nuclear fuel rods mounted in proximity with one another in a fuel assembly structure, through and over which coolant flows. The fuel rods are spaced from one another in co-extensive parallel arrays. Some of the neutrons and other atomic particles released during nuclear decay of the fuel atoms in a given fuel rod pass through the spaces between the fuel rods and impinge on the fissile material in adjacent fuel rods, contributing to the nuclear reaction and to the heat generated by the core.
Movable control rods are dispersed through the core to enable control of the overall rate of the fission reaction, by absorbing a portion of the neutrons passing between fuel rods, which otherwise would contribute to the fission reaction. The control rods generally comprise elongated rods of neutron absorbing material and fit into longitudinal openings or guide thimbles in the fuel assemblies running parallel to and between the fuel rods. Inserting a control rod further into the core causes more neutrons to be absorbed; and retracting the control rod reduces the extent of neutron absorption and increases the rate of the nuclear reaction and the power output of the core.
To monitor the neutron activities and coolant temperature within the core fuel assemblies, moveable in-core instrumentation has been employed in the past, such as moveable neutron detectors, that conventionally enter the core from penetrations in the bottom of the vessel. Additionally, fixed in-core neutron detectors have been employed that enter the core through the bottom of the reactor vessel and reside in the fuel assemblies during normal operation. In addition to fixed in-core instrumentation that enter through penetrations in the bottom of the vessel, there are fixed in-core instrumentation that enter through penetrations in the top of the vessel. In a few instances in the past, leakage occurred at the penetrations at the bottom of the vessel which presented significant repair problems. It soon became apparent that it would be desirable to have all the in-core instrumentation access the core from above. In this latter configuration, each in-core instrument thimble assembly is totally enclosed in a guide path composed of tubing which extends axially through the upper internals. The lower portion of this guide path extends down into the instrumentation thimbles within the fuel assemblies. However, even the fixed in-core neutron detectors, and the thermocouple assemblies that are used to monitor temperature within the core, have to be withdrawn from the fuel assemblies before the reactor core can be accessed for refueling operations. Thus, it is therefore necessary to provide a structure which can satisfactorily guide and protect the in-core instrumentation entering from the top of the vessel and mitigate the potential for leakage while enabling access for refueling.
These objectives have become even more of a challenge for some small modular reactor designs such as the one being proposed by Westinghouse Electric Company LLC, Cranberry Township, Pa., in the 200 megawatt class. The small modular reactor is an integral pressurized water reactor with all primary loop components located inside the reactor vessel. The reactor vessel is surrounded by a compact high pressure containment. Due to both the limited space within the containment and the low cost requirement for integral pressurized light water reactors, the overall number of auxiliary systems needs to be minimized without compromising safety or functionality. For example, the compact, high pressure containment associated with the design of small modular reactors does not allow for the incorporation of a large floodable cavity above the reactor vessel in which the transferred components can be shielded. Furthermore, in most traditional pressurized water reactors, the in-core instrumentation is retracted from the core prior to refueling. This is done by breaking primary pressure boundary seals and pulling the instrumentation through a conduit tube. This procedure is straightforward in plants with bottom mounted instrumentation since the conduit just extends from the bottom of the reactor vessel to a seal table located in a room separated from the reactor. In plants with top mounted instrumentation, this procedure is much more challenging because of the upper internal structure. This is further complicated when top mounted instrumentation is considered for use in an integral pressurized water reactor of a small modular reactor system that has a heat exchanger and pressurizer integrally incorporated in the reactor head closure. Top mounted instrumentation is preferred in plants that use a severe accident mitigation strategy commonly referred to as in-vessel retention. This strategy requires that there are no penetrations in the lower portion of the reactor vessel.
U.S. patent application Ser. No. 13/457,683, filed Apr. 27, 2012, entitled “Instrumentation and Control Penetration Flange for a Pressurized Water Reactor,” assigned to the Assignee of this Application, introduced a removable annular seal ring between the reactor head closure and the pressure vessel flange for routing cabling from the control rod drives and core monitoring instrumentation through the reactor vessel pressure barrier. U.S. patent application Ser. No. 13/742,392, filed Jan. 16, 2013, entitled “Method and Apparatus for Refueling a Nuclear Reactor Having an Instrumentation Penetration Flange,” assigned to the Assignee of this Application, teaches one method of refueling such a reactor. Refueling is on the critical path of most outages in which it is a part and any means of making the method of refueling more efficient can substantially reduce the cost of such an operation to utility operators. Accordingly, further improvements in reducing the steps that have to be taken to remove the instrumentation from the core so that they can be removed with the upper internals and expose the fuel assemblies is desirable for both conventional reactors and integral modular reactors.
In conventional reactors, the in-core instruments are encased in a long stainless steel tube, referred to an outer sheath, typically 30 to 40 feet (9.1 to 12.2 meters) long and approximately ⅜ of an inch (9.5 millimeters) in diameter. The outer sheath contains the instruments and the instrument leads. These lead wires extend the full length of the instrument and are terminated at one end in an electrical connector. The assembly of the instruments, instrument lead wires, outer sheath and electrical connector is called an in-core instrument thimble assembly. In the reactor, the end of the in-core instrument thimble assembly that has detectors in it, extends from the top of the fuel assembly, to almost the bottom, a distance in a conventional assembly of typically between 10 and 12 feet (3.05-3.66 meters). The non-active end of the in-core instrument thimble assembly contains lead wires that transmit the signal from the detectors to an electrical connector. In existing applications, the outer sheath of the in-core instrument thimble assembly passes through a vessel penetration. In more recent designs the penetration is usually in the reactor vessel's closure head, and the electrical connector is located outside of the reactor.
During a reactor refueling the in-core instrument thimble assemblies must be removed from the core to allow fuel repositioning. Some plant designs have an instrumentation grid assembly plate inside the reactor on an upper portion of the upper internals to which all of the in-core instrument thimble assemblies are attached. During the refueling, the instrumentation grid assembly plate is lifted and all of the in-core instrument thimble assemblies are withdrawn simultaneously from the reactor core. Other plants, that do not have an instrumentation grid assembly plate, withdraw each in-core instrument thimble assembly individually a sufficient distance to allow fuel movement. The withdrawn portion of the in-core instrument thimble assembly must be supported by an external means. Any change in structure of the in-core instrument thimble assemblies or the upper internals that will reduce the number of steps required to withdraw the in-core instrument thimble assemblies from the core will reduce the critical path refueling time and minimize the possibility of damaging the in-core instrument thimble assemblies due to a mishandling error. This is especially true in the crowded environment of a small, integral modular reactor.
According, it is an object of this invention to modify the in-core instrument thimble assemblies in a way that will minimize the number of steps required to withdraw the in-core instrument thimble assemblies into the upper internals and remove the upper internals from above the reactor core.
It is a further object of this invention to provide such a modification that will minimize the number of times submerged electrical connectors need to be disassembled.